Maybe instead of putting him flat on his belly, put him tilted over more to one side. 4 months is still on the earlier side for rolling over. Don't worry until he's 6 months and not trying to roll over. He might not be ready yet. Give it a few weeks and try again later. :)
My son didn't start rolling until almost 5 months so don't worry! Those few extra weeks make a difference too! Try putting him on his belly to start, with a rolled up receiving blanket under his chest to keep him off the floor a little. Couldn't hurt. I have heard that helps them enjoy floor time better, my sister did it that way.
I wouldn't worry about it. My daughter will be 5months this Tuesday and JUST started rolling over. She HATED tummy time, so i started to do tummy time on her bobby pillow. Once she started to enjoy tummy time on the pillow, I moved her to her belly on the floor. I also started to put her on her side and distract her with toys. Now, I can't put her on her back or her stomach without her rolling over. :)
my daughter does the same thing, when she was 3 months i started to lay her down for tummy time and she would roll over but when she wants to roll over to her belly she cant she cries and screams so i go and help her roll over. i think its still early so lets just wait and see.